In the related art, e.g., in a semiconductor manufacturing line configured to manufacture semiconductor devices, a number of semiconductor manufacturing apparatuses have been arranged in a clean-room under a clean atmosphere. Inside the clean-room, a down flow of clean air is formed to flow from the ceiling toward the floor of the clean-room. Thus, dust found in the clean-room is allowed to be discharged out of the clean-room.
In order to form a down flow in such clean rooms, a grating having a number of apertures is placed in the floor of the clean-room. A space in which the down flow is distributed is placed below the grating. The grating can be removed. A workman can, for example, enter the space below the grating to perform work when the grating is removed.
However, since an opening is exposed in the floor when the grating is removed, there is a potential danger that the workman may fall into the space through the opening. To avoid this danger, attempts have been made to warn the workman by setting up a pylon or barricade tapes around the opening when the grating is removed. In addition, in order to prevent workmen from falling into openings on construction sites and so on, there has been proposed a device for supporting baseboards on or around such openings.
In such a device, a warning sign for a workman was provided by setting up a pylon or barricade tapes around the opening when the grating in the clean-room was removed. However, a pylon or barricade tapes may not be promptly set up right after removing the grating and the workman may not be aware of the existence of the opening at that time. Thus, an accident where the workman could fall into the opening is possible at that time. In addition, even if a pylon or barricade tapes are set up, the workman may not be aware of the existence of the pylon or barricade tapes. Thus, there is another possibility of an accident occurring where the workman could fall into the opening.